If you really want to go green, the conventional thinking goes, buy a hybrid. Practically speaking though, there is a faster and cheaper option: shift to a low-carbon diet. The meal plan of the average American family accounts for 2.8 tons of CO2 emitted annually, compared with 2.2 tons for driving. Worldwide agriculture contributes some 30% of global greenhouse-gas emissions, far more than transportation. So when it comes to cutting your carbon footprint today, the truth is that what you eat is as important as what you drive. "If you can't buy a Prius," says Jonathan Kaplan of the Natural Resources...